Acoustic wave devices using piezoelectric thin film resonators have been used for filters and multiplexers of wireless devices such as, for example, mobile phones. The piezoelectric thin film resonator has a structure designed to have a lower electrode and an upper electrode facing each other across a piezoelectric film as disclosed in Japanese Patent Application Publication No. 2001-24476 (hereinafter, referred to as Patent Document 1). The region where the lower electrode and the upper electrode face each other across the piezoelectric film is a resonance region. An air gap or an acoustic mirror is located under the resonance region so as to prevent the restriction of the oscillation. The air gap or the acoustic mirror is formed so as to include the resonance region in plan view.
It has been known to arrange a plurality of piezoelectric thin film resonators so as to be included in a single air gap in plan view as disclosed in, for example, Patent Document 1. It has been known that adjacent piezoelectric thin film resonators that do not share the air gap share the lower electrode as disclosed in, for example, Japanese Patent Application Publication No. 2013-38658 (hereinafter, referred to as Patent Document 2). It has been known to form an insertion film in an outer peripheral region in the piezoelectric film as disclosed in, for example, Japanese Patent Application Publication No. 2014-161001 (hereinafter, referred to as Patent Document 3). It has been known that a resonator is divided so as to have the polarization direction of the piezoelectric film in which the secondary distortion is reduced as disclosed in, for example, Japanese Patent Application Publication Nos. 2008-85989 and 2009-10932 (hereinafter, referred to as Patent Documents 4 and 5).
For example, when a resonator is divided so as to reduce the secondary distortion as described in Patent Documents 4 and 5, it may be considered to share the lower electrode of the adjacent piezoelectric thin film resonators as described in Patent Document 2. However, when the lower electrode or the like connecting the adjacent piezoelectric thin film resonators is long, the secondary distortion is not completely cancelled between the divided resonators. When a plurality of piezoelectric thin film resonators are located in a single air gap as described in Patent Document 1, the distance between the piezoelectric thin film resonators can be shortened. However, since a complicated structure is provided in a single air gap, the multilayered film or the like may be damaged. For example, cracks may be formed.